Electron beam tube



Oct. 4, 1960 L.. s. NERGAARD ELECTRON BEAM TUBE:

Original Filed Jan. '7, 1949 INVENTOR. Eo/v 5. NERGAARD BY f f Trax/Vir United States Patent() ELECTRON BEAM TUBE Leon S. Nergaard, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Original application Jan. 7, 1949, Ser. No. 69,6342 now Patent No. 2,782,339, dated Feb. 19, 1957. Drvlded and this application Feb. 18, 1957, Ser. No. 640,906

6 Claims. (Cl. S15-3.6)

This invention relates to coupling devices for electron beam tubes, such as traveling wave tubes.

The present application is a division of my copending application Serial No. 69,634, tiled January 7, 1949, now U.S. Patent No. 2,782,339, issued February 19, 1957, assigned to the same assignee.

Said copending application discloses and claims a traveling wave amplifier tube wherein a signal is amplified by means of longitudinal or plasma oscillations in two electron beams of different average electron velocities. When the relative plasma frequencies and relative average electron velocities in the two beams are properly adjusted, the interaction between the two beams leads to a growing wave. Such a tube requires an evacuated envelope containing suitable electron gun structure at one end thereof for producing two electron beams in the same general direction through an elongated drift tube, input coupling means near the gun structure for coupling an input transmission line through the envelope to the beams for modulating the latter with an input signal to be amplified, and output coupling means located beyond the drift tubes for coupling the beams to an output transmission line.

An object of the present invention is to provide a beam tube with new and improved means for coupling the electron stream to an external input or output line.

Another object is to provide a coupling means having a bandwidth limited substantially only by the electronic bandwidth of the tube itself.

In accordance with the invention, the electron beam in a beam tube, such as a traveling wave tube, may be coupled to an external input or output transmission line by means of a helical inner conductor surrounding the beam path within an insulating envelope portion and a tapered hollow outer conductor surrounding the envelope portion and at least part of the inner conductor and connected at its smaller end to the inner conductor. The outer conductor may be a tapered metallic helix or an exponential metal cone.

In the annexed drawings:

Fig. l is a plan view in axial section of a traveling wave tube embodying one form of the invention;

Fig. 2 is a schematic diagram of a circuit for the tube shown in Fig. 1; and

Fig. 3 is a plan view in axial section of a modification of the tube shown in Fig. 1.

In the embodiment of the invention shown in Figs. l and 2 the signal to be amplified is impressed on the two beams by velocity modulation and the tube employs a novel output coupling means or system which permits exploiting the large electronic bandwidth of the tube.

Referring to Fig. l, a cup-shaped insulating member 60 containing an electron gun structure is coaxially joined, by a metallic sealing ring 61, to a relatively short cylindrical vmetallic tube 62 which, in turn, is joined to an elongated cylindrical metallic drift tube 64 by metallic sealing rings 65 and 66 and an insulating ring 67. The ring 65 may be an extension of the ring 61, as shown.

The outer end of the drift tube 64 is joined to a cupshaped metallic anode or collector 68 by means of two spaced metallic sealing rings 69 and an intermediate elongated insulating tube 70 which forms part of the novel output system to be described later.

The gun structure comprises two cathodes 72 and 73 in the form of spaced concentric annular segments of a spherical surface having its center along the tube axis, cathode heaters (not shown) and a single grid structure. The grid structure is made up of a mounting ring 75 to which are attached uniformly spaced grid wires 76 which lie in a spherical surface concentric with that of the two cathodes. The two cathodes and the single grid are maintained in spaced relation and are mounted on the button stem by stem leads 78, supporting rods 79 and nsulating beads 80. The stem leads 78 provide means for making separate direct current connections to the cathodes, heaters and the grid.

The input signal is impressed on the beams by means of an annular cavity resonator structure 82 surrounding the gap 83 between the tubes 62 and 64. This resonator structure is formed by the adjacent ends of the tubes 62 and 64, a pair of flanged rings 84 and 85, a cylindrical shell 86 and an annular tuning slider 87. A coaxial line coupling loop 88 is provided to introduce the signal t0 the resonator.

The novel output system of the invention as shown in Fig. 1 comprises two helical conductors in the form of coextensive metallic helices 90 and 91 positioned coaxially inside and outside of the insulating tube 70. The inner helix 90 is positioned close to the tube 70 and connected between the two rings 69, and has uniform diameter and pitch throughout its length to give uniform axial phase velocity. The outer helix 91, which is connected to the rst ring 69 and to the output line 92, is tapered along its length in such manner that the ratio of the diameters b and a of the two helices varies with distance x along the length of the lline formed by the two helices as follows:

in which v/c is the ratio of the phase velocity along the line -to the phase velocity in free space, L is the length of the line, and Z0 is the surge impedance of the line at x=L. Both helices are proportioned so that the axial phase velocity of wave propagation therealong is approximately equal to the average velocity of the electrons in the beams. In theory, this coupling network: (l) has an axial phase velocity equal to the average velocity of the electrons in the beams, so that energy may be extracted continuously from the stream over a long distance; (2) displays a constant impedance to the beams, of a character appropriate to the extraction of energy therefrom; and (3) presents a resistance to the output line at its output terminals, so that if the surge impedance of the line is made equal to this resistance no reflection will occur. The result is an output system which, in theory, has a bandwidth limited only by the electronic bandwidth of the tube. Because it is impossible to build a tube and circuit which conforms exactly to theoretical conditions, or to formulate an exact theory for a practical system, the practical performance will be somewhat short of the theoretical performance. However, the bandwidth obtainable with this coupling system is expected to far exceed that of currently used coupling systems.

It should be noted that the output system shown in Figs. 1 and 2 is not limited to the particularvuse in a plural beam tube as shown, but instead, may be applied to conventional traveling wave tubes employing a single electron beam. Moreover, a similar coupling system may be used as an input system in place of the input system of Figs. 1 and 2, or in conventional traveling wave tubes.

Fig. 3 shows another embodiment of the invention which has the new and improved coupling system at both the input and output ends of the tube. The electron gun structure is the same as that of Fig. 1. The sealed enclosure is formed by the cup-shaped insulating member 60, sealing ring 61, a relatively short flanged metal tube 101, an insulating tube 102, a metal drift tube 104 anged at both ends, a second insulating tube 105 and a anged metal anode or collector 106. The input coupling system comprises a uniform metal helix 108 coaxially disposed inside the tube 102 and having uniform pitch and diameter such that the axial phase velocity of the wave along the helix is equal to the average velocity of the electron beams, and an exponential metal cone 109 coaxially disposed outside the tube 102 coextensive with the helix 108. The cone is tapered to conform to the relation Zovx Lesa-r. a

given above in connection with Fig. l. The smaller end of the cone 109 is clamped to one flange oi the drift Vtube 104 by means of rings 110 and 111 and the other end is grounded through a metal ring 112 to the tube 101. The helix 108 is connected at its output end to the drift tube 104 and has its input end brought out through the insulating tube 102 to an input coaxial line 114. The output coupling system is similar to the input system except that the output terminal of the helix 115 is connected to the anode 106, and the anode and the exponential cone 116 are connected to the inner and outer conductors 117 and 118, respectively, of a coaxial output line extending along the axis of the tube.

Although three specific embodiments of the invention have been described for purposes of illustration, it will be appa-rent that many variations may be made in the particular structures employed without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

l. A beam tube comprising an elongated envelope including a hollow insulating portion, an electron gun structure in said envelope for projecting a beam of electrons along a beam path extending through said insulating portion, and means for coupling said beam through said portion to an external transmission line; said means comprising a helical inner conductor having a predetermined uniform axial phase velocity for waves propagated therealong positioned within said insulating portion, a tapered diameter hollow outer conductor surrounding said insulating portion along at least part of the length of said inner conductor, external terminal means connected to the larger end of said tapered outer conductor and to the end of said inner conductor adjacent thereto and adapted to be connected to an external transmission line, and low impedance means connecting the smaller end of said tapered conductor through said envelope to said inner conductor.

2. A beam tube as in claim l, wherein said outer conductor is substantially coextensive axially with said inuell conductor.

3. A beam tube comprising an elongated envelope including a hollow insulating portion, an electron gun structure in said envelope for projecting a beam of electrons along a beam path extending through said insulating portion, and means for coupling said beam through said portion to an external transmission line; said means comprising a helical inner conductor having a predetermined uniform axial phase velocity for waves propagated therealong positioned within said insulating portion, a tapered diameter hollow outer conductor surrounding said insulating portion along at least part of the length of said inner conductor, external terminal means connected to the larger end of said tapered outer conductor and to the end of said inner conductor adjacent thereto and adapted to be connected to an external transmission line, and low impedance means connecting the smaller end of said tapered conductor through said envelope to said inner conductor, said helical conductor having uniform diameter and pitch, and the diameter of said tapered outer conductor varying as follows:

zgvx 0:61am

where b and a are the diameters of the outer and inner conductors, respectively, v/c is the ratio of the phase velocity along the line formed by said conductors to the phase velocity in free space, L is the length of said line and x is the distance therealong, and Z0 is the surge impedance of said line at x=L.

4. A beam tube as in claim 3, wherein said outer conductor is a tapered helical conductor having an axial phase velocity substantially equal to said axial phase velocity of said helical inner conductor.

5. A beam tube as in claim 3, wherein said outer conductor is an exponential cone.

6. A beam tube comprising electron gun means for projecting a beam of electrons along a given path, and means for coupling said beam to an external transmission line; said coupling means comprising a helical inner conductor having a predetermined uniform axial phase velocity for waves propagated therealong surrounding and extending along a portion of said path, a tapered diameter hollow outer conductor surrounding and spaced from said inner conductor along at least part of the length thereof, a pair of terminals connected respectively to the larger end of said tapered outer conductor and to the end of said inner conductor adjacent thereto and adapted to be connected to an external transmission line, and low impedance means connecting the smaller end of said tapered conductor to said inner conductor.

References Cited in the le of this patent UNITED STATES PATENTS Re. 23,647 Lindenblad Apr. 2l, 1953 2,615,141 Hansell Oct. 2l, 1952 2,823,333 Quate Feb. Il, 1958 2,850,704 Munushian Sept. 2, 1958 FOREIGN PATENTS 1,043,066 France June l0, 1953 955,975 Germany Jan. 10, 1957 

